Adding shunt capacitor and resistor in zobel causes instability - Any ideas why?

[DF96]

You are right: a capacitive load can sometimes result in a negative impedance. Perhaps the Vbe multiplier itself needs a little damping? It uses a significant amount of NFB within itself to get a constant output voltage, so there is always scope for instability.

 

[AndrewT]

so it comes back to what I have read.
Using a higher esr electrolytic rather than a very low esr film may provide damping that avoids ringing/oscillation.